Vernier Calliper is a simple device to measure very small lengths easily. The movable cylindrical scale is calibrated into 50 units. If it is rotated two full rounds, it moves 1mm along the fixed scale.

So,

30 Vernier units = 29mm
1 Vernier unit = 29/30mm

Therefore, the Vernier scale lets us measure us small lenghts. The following animation describes the working of the device.

Play with the device by clicking the arrow-heads. When you want to take the reading, please click the '?' symbol. Then the answer will appear at Text label.